Tektronix TPS2000 Series User Manual page 130

Math FFT
The Math FFT function includes three FFT Window options. There
is a trade-off between frequency resolution and amplitude accuracy
with each type of window. What you want to measure and your
source signal characteristics will help you to determine which
window to use.
Window
Hanning
Flattop
Rectangular
FFT Aliasing
Problems occur when the oscilloscope acquires a time-domain
waveform containing frequency components that are greater than the
Nyquist frequency (refer to Nyquist Frequency on page 5- - 3). The
frequency components that are above the Nyquist frequency are
undersampled, appearing as lower frequency components that "fold
back" around the Nyquist frequency. These incorrect components are
called aliases.
5- 8
Measure Characteristics
Periodic Better frequency, poorer magnitude
waveforms accuracy than Flattop
Periodic Better magnitude, poorer frequency
waveforms accuracy than Hanning
Pulses or Special-purpose window for waveforms that
transients do not have discontinuities. This is essen-
tially the same as no window
TPS2000 Series Digital Oscilloscope User Manual